Medium to high density polyethylene (PE) resins having a bimodal or multimodal molecular weight distribution are known and widely utilized within the film industry. The use of PE resins having at least two different molecular weight components and/or components which differ structurally from each other is particularly desirable for the production of films where, by judicious selection and design of the resin components, film properties and processability can be balanced.
Multimodal high molecular weight, high density polyethylene (HMW HDPE) resins are particularly useful for the production of thin films such as those used for tee-shirt bags, merchandise and produce bags and the like. Such thin films are typically blown using high stalk or high neck processes as described in U.S. Pat. No. 6,552,150. The resins employed must not only be capable of being drawn down into thin gauges, but must also have the ability to be extruded at high rates. The latter, i.e., film output, is often limited by either stability of the blown film bubble or by pressure limitations in the extruder.
Multimodal PE resins and processes for their preparation in multiple reactor systems are known. For example, a two-stage ethylene polymerization process wherein different catalysts are employed in the first and second stages is described in U.S. Pat. No. 5,747,594. A single-site catalyst is utilized in the first stage and a Ziegler catalyst is used in the second stage.
A dual reactor process for the polymerization of ethylene using single-site catalysts but with different activators in the first and second reactors is disclosed in U.S. Pat. No. 6,372,864.
Whereas processes of the above types yield resins comprised of polymers of differing molecular weights, the use of different types of catalysts and/or different activators presents operational problems.
U.S. Pat. No. 6,995,216 discloses a process for the production of broad molecular weight distribution or bimodal HDPE, linear low density polyethylene (LLDPE) and medium density polyethylene (MDPE) resins using a bridged indenoindolyl ligand-containing Group 4 transition metal complex and an activator. The process can be carried out in multiple stages or in multiple reactors. The same complex and activator are used in all stages or reactors.
Multimodal PE resins and processes for their preparation using two or more single site catalysts are disclosed in published U.S. patent application 2007/0055021A. The reference generally discloses the polymerization of ethylene and an α-olefin comonomer in either single or multiple reactor(s) arranged in parallel or in series to form a multimodal HDPE or MDPE in one stage and a multimodal LLDPE or ultra low density polyethylene (ULDPE) in a second stage. In one embodiment the single site catalysts used are indenoindolyl catalysts and in another embodiment a mixture of bridged and non-bridged indenoindolyl catalysts is employed.
We have now unexpectedly discovered a process whereby multimodal PE resins which have a unique balance of properties rendering them useful in high stalk blown film processes are produced utilizing a specific combination of bridged and non-bridged indenoindolyl catalysts.